The operation of a gas turbine engine involves compressing air in a series of airfoils mounted on disks which are alternately rotating and stationary, combining the compressed air with fuel in a combustion operation, and extracting power by expansion in a second series of airfoils mounted on disks which are alternately rotating and stationary.
In order to maximize efficiency and power output, it is necessary to reduce leakage of the compressed air and combustion products past the tips of the rotating airfoils. An airseal is typically formed by bonding a layer of abradable material to a circular support ring. An alternate method is to bond a honeycomb structure to the backup ring and pack abradable material in the cells of the honeycomb material.
At initial assembly of the engine, the inside diameter of the surface of the abradable material is less than the outside diameter of the tips of the airfoils attached to the rotating disks. During the initial operating period of the engine, the tips of the airfoils remove some of the abradable material from the inside surface of the airseal until the inside diameter of the airseal abradable material is virtually the same as the outside diameter of the rotating airfoils. This insures the least possible gap between the tips of the airfoils and the airseal, and minimizes leakage past the tips of the airfoils during engine operation.
After a designated period of service time, the engine is removed from service and the necessary overhaul and repair operations are performed to insure continued safe operation of the engine. During overhaul and repair, it is necessary to remove the old abradable material and replace it with new abradable material so that a new airfoil tip path may be formed in the airseal after reassembly of the engine.
At overhaul and repair, the airseals often deviate from a true circular configuration. This can be due to deviations from the nominal configuration in the original part or to distortion created by exposure to the elevated temperature conditions during engine operation. During overhaul and repair, the abradable material is typically removed from the support ring using conventional machining techniques. This includes fixturing the support ring in a manner so as to force the ring to a more nearly circular configuration, and rotating the ring about its center past a single point cutting tool which removes the abradable material. This technique generally involves extensive setup and machining time, as well as complex and expensive fixturing, and can still be inaccurate enough to either fail to completely remove the abradable material or to inadvertently cut into the support ring.
In U.S. Pat. No. 5,055,752 to Leistensnider et al and of common assignee with this application, a method is presented for machining the elongated edge of a workpiece to preselected dimensions and tolerances using a numerically controlled machining system. The surface of the workpiece along the length of the edge to be machined is probed to determine edge dimensions and/or the actual position and orientation of the edge at preselected locations. The data generated are stored and the edge of the workpiece is machined under the direction of a machine program which accesses the measured data, compares it with original design data for the part, and machines the elongated edge of the part in accordance with the original design configuration as adjusted for the measured data.
Since the conventional machining techniques are inherently inaccurate and the technique disclosed in Leistensnider et al is restricted to a workpiece which has an elongated edge, what is needed is a method to remove a layer of material from a circular object which is slightly out of round so as to leave a predetermined amount of material on the circular object. As applied to a gas turbine engine airseal, this amounts to removing the abradable seal materials completely without inadvertently removing any of the support rings which support the abradable seal materials.